Castañeda was appointed Provost Marshal General of the Philippine Army on 1 June 1946. He became Chief of Staff of the Armed Forces of the Philippines two years later. The night before the Philippine Parity Rights plebiscite in 1947, President Manuel Roxas addressed a rally at Plaza Miranda. He narrowly avoided an assassination attempt by Julio Guillen, a disgruntled barber who threw a grenade on the speaker's platform. José Avelino, the Senate President, saw the bomb and gave it a kick. Castañeda then kicked it further down a set of steps as he covered the President with his own body. The grenade exploded near the audience, killing two people and injuring others. Guillen was arrested and executed via the electric chair on 26 April 1950. Castañeda was conferred the Medal of Valor as a consequence of his actions that night.
Medal of Valor citation
"By direction of the President, pursuant to paragraph 2a, Section I, AFPR 600-45, dated 16 December 1948, the Medal for Valor is hereby awarded to:
MGEN MARIANO N CASTAÑEDA O-1032 AFP
for conspicuous gallantry and intrepidity in the face of imminent danger at the risk of life to protect His Excellency, Manuel A Roxas, the first President of the Republic of the Philippines.
On the event of the Party Plebiscite on 10 March 1947, after the late President Roxas had delivered his forceful speech for its approval to about 25,000 people assembled in Plaza, Miranda, Quiapo and to the nation over the National Radio Network, an attempt to assassinate him was made by means of a hand grenade thrown at the President. The deadly missile landed on the speaker’s platform and rolled towards the center of the late President Roxas and other ranking government officials. In spite of the inevitable explosion and its lethal results, General Castañeda, then Chief of the Constabulary, in complete disregard of his personal safety, rushed from his seat behind the President’s chair to the lethal weapon which was about to explode, and with extraordinary coolness and presence of mind ordered the people to lie down and then kicked the death-dealing grenade down the steps of the platform where it exploded. His presence of mind and display of exemplary courage and bravery in the timely disposal of the lethal grenade saved the life of the First President of the Philippines and those of his family and other higher ranking officials of the Republic, who at that moment, were all with him on the platform.."
Death
Mariano Castañeda died in New York City at the age of 77 on 8 September 1970, leaving behind his wife and six children.